1. Field of the Invention
The present invention relates to an image fixing method and apparatus for obtaining a full-color toner image by the electrophotographic process, or electrostatic image recording and printing method.
2. Discussion of Background
In the electrophotographic image formation process, a contact heating image fixing method is widely employed to fix a developed toner image on an image receiving sheet. Namely, the toner image transferred to a sheet is fixed thereon by bringing the toner into direct contact with a heater. In the contact heating image fixing method, it is desired to achieve satisfactory image fixing performance at low temperature and prevent the off-set phenomenon from occurring even at elevated temperature.
In recent years, the trend from monochromatic image formation to full-color image formation has been rapidly developing in the field of copiers and printers. A market for full-color copiers and printers has been expanding.
In the full-color image formation using color toners, when a plurality of color toners is superimposed, each of the color toners is sufficiently melted to be mixed together. High glossiness is also desired in the full-color image formation. For obtaining full-color toner images with high glossiness, it is proposed to decrease the image fixing speed, or increase the image fixing temperature as disclosed in Japanese Laid-Open Patent Applications 9-258592, 9-190107, 9-106210, 5-119651, 5-158364, 5-16026, 9-265243, 10-153923, 10-282822, and 11-24463.
In the image fixing method by contact heating, a fixing method by use of a heat-application roller is conventionally prevalent because of its simple structure and easy handling. However, there is the drawback that a certain time is always necessary until the heat-application roller reaches a predetermined image fixing temperature. In addition, there is a risk that the temperature of the heat-application roller varies when the image-bearing sheet passes therethrough, or according to other external factors. As a result, the image fixing properties become poor and the off-set phenomenon takes place. To maintain the heat-application roller at an optimum temperature, the heat capacity of the heat-application means such as a heat-application roller is required to increase.
The above-mentioned problem is serious especially in the fixing of full-color toner image. In general, a color image appears clearer when the glossiness of the color image becomes higher. To increase the glossiness of the fixed color toner image, the surface smoothness of the fixed image is increased by the application of a large pressure thereto. Therefore, the surface portion of a heat-application roller is required to have a relatively thick elastic member.
Furthermore, since a typical full-color toner has a low viscosity, the problems of off-set phenomenon and winding of the image-bearing sheet around the roller occur resulting from the curvature of the heat-application roller. To solve these problems, coating of a release oil over the surface of the heat-application roller is proposed. For this proposal, however, a tank for the release oil becomes necessary.
To solve the above-mentioned problems caused by the roller fixing method, a belt fixing method is proposed. Further, there are proposed image fixing methods using a belt with no oil being coated thereon, or a slight amount of oil being coated thereon. As disclosed in Japanese Laid-Open Patent Applications 2-160259 and 2-161462, the belt fixing methods are proposed in consideration of the combination of the belt fixing method and a toner composition suitable for the belt fixing.
However, the heat-application roller is usually used for fixing the full-color toner image because a high pressure can be applied to the full-color toner image. In contrast to this, the pressure applied to the full-color toner image by the belt is too low to obtain a full-color image with high glossiness. Namely, the belt fixing has no pressure-application effect on the formation of full-color toner images with high surface smoothness, that is, high glossiness in practice.
To obtain high glossiness under such conditions, the toner composition with a low melt viscosity is used. However, occurrence of the off-set phenomenon becomes more frequent. Further, the surface condition of the belt largely affects the surface smoothness of the fixed full-color toner image.
For obtaining high glossiness of full-color toner images by the belt fixing, Japanese Laid-open Patent Application 2-160250 proposes to diminish the irregularity of a toner-image-deposited surface on an image receiving sheet by controlling the volume mean diameter of toner particles, and contents of fine grains and coarse grains in the toner. However, in this case, a glossy full-color toner image cannot always be provided. In Japanese Laid-Open Patent Application 11-125948, the surface roughness of a toner image formed on an OHP sheet is specified. Even though the surface roughness of the toner image on the OHP sheet is specified, the off-set properties and the glossiness are not always satisfactory when the same toner image is formed on a sheet of plain paper since the surface conditions between an OHP sheet and a plain paper are different.
As mentioned above, full-color toner images have been required to have high glossiness. However, in some cases, too glossy full-color toner image may not be preferred, for example, as an image output from a printer. On the contrary, a full-color toner image having no glossiness may be desired. In other words, there is an increasing demand for variation in glossiness of the full-color toner image. To meet this demand, Japanese Laid-Open Patent Application 4-194967 proposes an image fixing mode capable of selectively switching the glossiness of the obtained full-color toner images, that is, selectively producing a highly glossy image or an image with no glossiness using the same image fixing apparatus. However, the conditions for producing highly glossy toner image are disadvantageous for prevention of the off-set phenomenon. Depending on the conditions, the off-set phenomenon cannot be effectively prevented. The off-set phenomenon may occur in a halftone image portion, which will be hereinafter referred to as xe2x80x9cslight off-set phenomenonxe2x80x9d although a solid image portion is not impaired by the off-set phenomenon. Furthermore, when the difference in the levels of glossiness is too large in the above-mentioned image fixing apparatus, there is the risk that each glossiness level becomes unstable.
It is therefore a first object of the present invention to provide a contact heating image fixing method for producing fixed full-color toner images which can selectively exhibit different glossinesses. Namely, a first object is to selectively produce a full-color toner image fixed with no glossiness or low glossiness, capable of exhibiting sufficient color development properties no matter whether the toner image is a single color image or mixed color image, and a full-color toner image fixed with a glossiness ranging from an intermediate level to a high level, capable of preventing the off-set phenomenon from occurring.
The first object of the present invention can be achieved by a method for fixing full-color toner images on an image receiving member so that the fixed toner images selectively exhibit different glossinesses, comprising the step of bringing the toner images transferred to the image receiving member into contact with a heater with linear speeds being selectively switched in such a manner that the fixed toner images exhibit a mean surface roughness in a range of 9.5 to 16 xcexcm at a linear speed (A) at an image fixing temperature (T), and the fixed toner images exhibit a mean surface roughness of less than 8 xcexcm at a linear speed (B) at an image fixing temperature (Txe2x80x2), wherein the linear speed (A) is in a range of 100 to 300 mm/sec, the linear speed (B) is lower than the linear speed (A) by 25 mm/sec or more, and the image fixing temperatures (T) and (Txe2x80x2) are equal.
It is a second object of the present invention to provide an image fixing method for producing full-color toner images with color toners being fully mixed and fixed with high glossiness. To be more specific, the second object is to provide such full-color toner images fixed with high glossiness by use of an image fixing belt, without staining of the belt with toner resulting from the slight off-set phenomenon, and without the occurrence of the off-set phenomenon for an extended period of time, while a member for applying a slight amount of oil to the belt can be prevented from being contaminated with toner.
The second object of the present invention can be achieved by a method for fixing full-color toner images with high glossiness on an image receiving member, comprising the step of bringing the toner images transferred to the image receiving member into contact with an image fixing belt so that the fixed toner images exhibit a mean surface roughness of less than 8 xcexcm.
A third object of the present invention is to provide a contact-heating type image fixing apparatus for producing fixed full-color toner images capable of selectively exhibiting different glossinesses.
The third object of the present invention can be achieved by an apparatus for fixing full-color toner images on an image receiving member so that the fixed toner images selectively exhibit different glossinesses, comprising a heater for heating the toner images transferred to the image receiving member in such a configuration that the heater is brought into contact with the toner images, with linear speeds being selectively switched in such a manner that the fixed toner images exhibit a mean surface roughness in a range of 9.5 to 16 xcexcm at a linear speed (A) at an image fixing temperature (T), and the fixed toner images exhibit a mean surface roughness of less than 8 xcexcm at a linear speed (B) at an image fixing temperature (Txe2x80x2), wherein the linear speed (A) is in a range of 100 to 300 mm/sec, the linear speed (B) is lower than the linear speed (A) by 25 mm/sec or more, and the image fixing temperatures (T) and (Txe2x80x2) are equal.
A fourth object of the present invention is to provide a belt-type image fixing apparatus for producing full-color toner images with colortoners being fully mixed and fixed with high glossiness.
The fourth object of the present invention can be achieved by an apparatus for fixing full-color toner images with high glossiness on an image receiving member, comprising an image fixing belt for heating the toner images so that the fixed toner images exhibit a mean surface roughness of less than 8 xcexcm by bringing the belt into contact with the toner images transferred to the image receiving member.